1. Field of the Invention
The present invention relates in general to optical inspection devices and in particular to a method and apparatus for optically detecting checks in glass tubes which are being continuously moved through a production machine.
2. Description of the Prior Art
In the manufacture of glass tubes, tubing is cut to achieve a roughly desired length. During the cutoff operation, cracks may occur in the tube. Such cracks usually extend from the cut edge down into the body of the tube. When the ends of the tube are glazed or tooled, the crack may be partially or completely healed. However, if the original crack in the tube extends past the glazed or tooled area, it will remain unhealed as a fire check. Such fire checks are unacceptable defects in the finished glass tube product.
The use of optical scanning devices for inspecting the side walls of glass containers is well known. Numerous devices, such as those shown in U.S. Pat. Nos. 3,708,680 and 3,716,136, have circuitry which includes means for receiving and interpreting light passed through or directed onto an item under inspection. Such devices incorporate either a visual display for comparison of the item or employ a device capable of producing a resistance proportional to the intensity of the light directed thereon. Whether the output of such a device is visual or electrical in nature, it is eventually compared against a model to determine if the item under inspection is suitable as to size and construction and is without flaws, cracks, or foreign objects. Such devices are intended to provide an automated inspection means for checking single or multiple objects, such as in a moving column of bottles.
U.S. Pat. No. 3,877,821 discloses an apparatus having a scanning array which is serially interrogated to generate a train of pulses having amplitudes representing the light transmitted through an object under inspection. Adjacent pulses are compared to generate pulses having amplitudes which represent the difference in pulse amplitudes. The difference pulses can be utilized to indicate a defect in the object being inspected. U.S. Pat. No. 3,942,001 discloses an apparatus for detecting the presence of extraneous matter or cracks in translucent containers. A spot beam of light is projected through the container to generate an inspection signal which is compared with an acceptance signal. The acceptance signal amplitude is varied in accordance with the position of the spot beam with respect to the container.